This invention relates to the removal of sulfur compounds from gases, and more particularly, to a method and system for the removal of sulfur compounds from gases using sorbents and to the regeneration and recycling of spent sorbents in a moving bed process and system.
Gases which contain sulfur compounds, such as gases that are derived from coal gasification techniques, have been of considerable interest as a source of gas for use in various systems and industrial applications. Since such gases contain sulfur compounds, in order to meet environmental standards and to prevent damage to equipment, it is necessary to remove the sulfur compounds from the gases.
The sulfur compounds in such gases generally include primarily hydrogen sulfide (H.sub.2 S) and, in lesser amounts, carbonyl sulfide (COS) and the like. For example, untreated coal gas generally contains about 1,000 to about 6,000 ppmw of H.sub.2 S, COS and other compounds, depending on the sulfur content of the coal from which the coal gas is derived. At least 90 percent of these sulfur compounds should be removed to meet current environmental and emission standards for utility power generation operating with medium to high sulfur coal.
Integrated coal gasification/combined cycle (IGCC) power generation is an extremely attractive alternative for coal-based production of energy. In the IGCC process, coal is gasified with air (or oxygen) and steam to produce a combustible gas comprising principally carbon monoxide, carbon dioxide, hydrogen, methane, water vapor and nitrogen (when air is used as the oxidant), and also containing minor amounts of other gaseous species. The coal gas is purified to remove species such as particulate matter, alkali metals, and sulfur compounds, principally hydrogen sulfide as discussed above, which are harmful to the generating equipment and/or to the environment.
Coal-derived fuel gas leaves the gasifier at temperatures higher than 500.degree. C. Numerous methods for removing hydrogen sulfide at temperatures below about 200.degree. C. are commercially available and have been demonstrated for IGCC applications. However, since the coal-derived fuel gas is produced at significantly higher temperatures, large-scale cooling and processing are necessary to reduce the temperature of the coal-derived fuel gas to that required for these numerous prior art methods for desulfurization. Energy efficiency losses and significant process complexity and cost result from the necessity to cool the coal gas. The cooling, cleaning and reheating of the coal-derived fuel gas itself is thermodynamically inefficient and requires costly heat exchangers.
An alternative approach for cleaning the coal gas is to remove the hydrogen sulfide and other compounds at the coal gasifier exit temperature or with little or no cooling. This requires the development of desulfurization processes capable of operation at 500.degree.-700.degree. C. The desirable high temperature desulfurization process requires no cooling of the fuel gas, and therefore, the thermodynamic penalties and process complexities associated with low temperature desulfurization are avoided. Even though high sulfur removal efficiency has been achieved, hot coal-derived fuel gas desulfurization has failed to gain commercial acceptance because of potential high cost as well as numerous technical and operational drawbacks and disadvantages. In the prior art processes and systems, metal oxides such as iron, copper, zinc and vanadium oxides, mixed metal oxides such as zinc and copper ferrites and metal oxide blends have been found to be effective for this purpose. They have also been found to remove carbonyl sulfide (COS), the minor sulfur compound. These metal oxide species are referred to herein and in the appended claims as metal oxide sorbent, regenerated metal oxide, re-usable metal oxide, sorbent which reacts with sulfur compounds, and sorbent, all of which may be used interchangeably herein.
Hot gas desulfurization processes generally involve contacting the sorbent, such as a metal oxide, with the hot gas, such as hot coal gas, to form metal sulfides. The metal sulfides are referred to herein as sulfided sorbent, sulfur-rich metal sorbent, spent metal-sulfur compound, sulfur-rich sorbent, and spent sorbent, all of which may be used interchangeably herein. The net sulfur sorption reaction is: EQU H.sub.2 S+MO=MS+H.sub.2 O (I)
wherein M is the metal present in the sorbent; MO represents the metal oxide; and MS represents the metal sulfide. For the sake of simplicity, M is represented as divalent, but it will be apparent that metals in other valence states may also be employed. Also, since the actual proportion of sulfur in the metal sulfide usually varies from theoretical, these equations may not accurately represent the stoichiometry of the reactions.
Following the absorption reaction, that is, the reaction in the absorber, the sulfided sorbent is regenerated to recover metal oxide. It is apparent that during this regeneration, sulfur compounds will be evolved and must be controlled in order not to cause additional pollution problems. Many processes have been devised to recover such sulfur compounds and to produce such products as sulfuric acid, gypsum, ammonium sulfate, elemental sulfur, and the like.
The standard approach to the regeneration of this sulfided sorbent is oxidation or roasting, to yield the metal oxide and either elemental sulfur or sulfur dioxide. As used herein, the regenerated metal oxide is referred to as metal oxide, re-usable metal oxide, regenerated metal oxide, re-usable sulfur-depleted sorbent and regenerated sorbent, all of which may be used interchangeably herein. The regeneration reaction may be represented by the following equation: EQU 2MS+3O.sub.2 =2MO+2SO.sub.2 (II)
wherein M, MS and MO are as defined above.
In the prior art processes, the concentration of sulfur dioxide in the regenerator off-gas is ordinarily low, typically less than about 3% and may be removed by using conventional methods.
Depending on the regeneration conditions, the oxidation reaction in the regenerator may be accompanied by the undesirable formation of metal sulfate. This may be represented by the following reaction: EQU MS+2O.sub.2 =MSO.sub.4 (III)
Hot gas desulfurization processes have been developed mainly by or for the U.S. Department of Energy. Most prior art processes and configurations utilize two fixed bed reactors, each containing pellets of metal oxide or metal sulfide and each operating alternately in an absorption or regeneration mode. The prior art processes using fixed bed reactors are disadvantageous, not only for the reasons stated above, but also because they are of a non-continuous, non-steady state operation where the reaction zone is not confined to one location in the reactor but it moves as the reaction progresses. Furthermore, in the prior art systems, contaminated sorbent removal requires system shut down. Further, in the prior art fixed bed systems, pressure drop increases during the absorption reaction due to the accumulation of particulate matter in the bed. A non-steady flow of regenerator off-gas results in more costly sulfur dioxide treatment in many of the prior art fixed bed systems, and both reactors in the fixed bed systems must be designed to withstand both oxidizing and reducing atmospheres at the high pressure of the absorber and the high temperature of the regenerator in alternate modes of absorption and regeneration. In the prior art systems, it is necessary to cool the highly exothermic regeneration reaction, and this requires significant dilution of the regeneration gas by an inert gas, such as steam or nitrogen, thereby having thermal and economic penalties to the process. This dilution of the regeneration gas results in a low sulfur dioxide concentration and increases the cost of treatment of the regenerator off-gas. Furthermore, in the prior art processes, the oxidation of metal sulfides may be accompanied by undesirable formation of metal sulfate. The metal sulfate remains with the regenerated sorbent, and the metal sulfate subsequently decomposes under the reducing conditions in the absorption reaction, thereby releasing sulfur dioxide into the coal-derived fuel gas and reducing the overall sulfur removal efficiency.